A family of genes encoding putative RNA binding proteins has been identified in Drosophila. Three members of the family have been characterized, two in detail. The two well-characterized genes, Hrb98DE and Hrb87F, encode the major protein components of nuclear ribonucleoprotein complexes. These proteins are closely related in sequence and structure to each other and to the A and B group proteins of vertebrate hnRNP complexes. Both genes show a high level of expression maternally, reduced but significant expression during late larval and pupal development, and low levels of expression during the rest of development. In situ hybridization of gene-specific probes to developing embryos has shown that the early maternal expression is spatially uniform but later expression is localized predominantly to the nervous system and a few other tissues. Antisera produced against a fusion protein reacts with both the Hrb98DE and Hrb87F proteins and identifies >12 spots on two-dimensional immunoblots. Antisera specific for individual proteins have allowed the identification of many of the spots. The proteins migrate as an 50S ribonucleoprotein complex in sedimentation analysis, and the integrity of the complex is dependent on the integrity of the associated RNA, as expected for hnRNP proteins. The third gene, identified by the R16 cDNA clone, is more distantly related to the Hrb genes than they are to each other. Like the Hrb proteins, the N-terminal half of the R16 protein has two copies of the RNP motif, a conserved 80 amino acid motif found in many RNA binding proteins. The C-terminal half is distinct, and encodes a proline-rich domain. Northern blot analysis indicates that at least three transcripts are expressed, and their levels vary during development.